The experiments in Project I are directed at elucidating the role of (1) features of electrical stimulation, and (2) the condition of the stimulated neural elements, on hearing measured psychophysically. "Features of electrical stimulation" include stimulus waveform and duration, electrode- array configuration and placement, and new multielectrode current-focusing stimulation strategies (developed in Project II). "Condition of the stimulated neural elements" includes the number of neurons present in the stimulated tissue and the physical condition of those neurons. There are 7 specific aims. The first four aims focus on questions of what variables influence the behavioral detection of electrical stimuli. Psychophysical data will be collected for a variety of electrical signals under a variety of conditions of the stimulated neural array. These data, in conjunction with neural recordings (in collaboration with Dr. Charles Parkins at the LSU Medical Center) and neural models (Project III), will be used to formulate and test hypotheses about the mechanisms underpinning electrical- stimulus detection. Implications of our findings with regard to the use of detection thresholds for predicting the efficacy of auditory prostheses will be tested in implanted human subjects. Aims 5 and 6 are directed at variables that influence behavioral discrimination of site of stimulation, and Aim 7 will use contralateral masking to characterize the effects of the electrical stimuli at suprathreshold levels. the subjects for most of these experiments will be nonhuman primates (Macaque monkeys). A few experiments will be conducted with human subjects. Standard animal psychophysical procedures will be used in operantly conditioned monkeys. These procedures will be modified slightly for testing human subjects. Most of the subjects will receive stimulation via multielectrode arrays in the scala tympani of a deafened cochlea. Cochlear-wall implants will also be tested in monkey subjects and compared with scalar implants in the same subjects. After behavioral testing is completed, acute single-auditory- nerve-fiber recordings will be made in the monkeys to determine responses of nerve fibers at psychophysical-detection-threshold levels. All monkey subjects will be examined histologically at the end of the experiments to determine the locations of stimulating electrodes and the condition of the stimulated neural structures. These studies will yield data relevant to the design and calibration of processors for auditory prostheses, and relevant to preoperative testing of cochlear implant candidates to determine nerve survival patterns and their functional implications. The comparative studies will aid our understanding of how animal experiments are best interpreted in terms of application to patients. Finally, these studies will contribute to understanding the processing and perception of stimuli by the auditory nervous system.